1. Field of the Invention
The present invention relates to a method for compensating for wavelength dispersions caused when transmitting wavelength division multiplexed signal light using an optical transmission path having a plurality of repeating sections, and an optical transmission system adopting such a method, and particularly to a method for compensating for wavelength dispersions and an optical transmission system capable of reducing affections of cross phase modulation caused among optical signals of respective wavelengths.
2. Related Art
Conventionally, in an optical transmission system over a long distance, an optical signal has been transmitted making use of an optical regenerating repeater which converts an optical signal into an electrical signal, to thereby perform retiming, reshaping and regenerating. However, at present, the advancement of practical use of an optical amplifier has led to the investigation of an optical amplifying-and-repeating transmission system which utilizes an optical amplifier as a linear repeater. By replacing an optical regenerating repeater with an optical amplifying-repeater, it is expected that the number of parts within the repeater is remarkably reduced, to thereby ensure reliability and to permit cost reduction. Further, as one method for realizing a large capacity of an optical transmission system, attention has been directed to a wavelength division multiplexing (WDM) optical transmission system which multiplexes two or more optical signals having wavelengths different from each other to transmit in a single optical transmission path. In a WDM optical amplifying-and-repeating transmission system provided by combining the aforementioned optical amplifying-and-repeating transmission system with the WDM optical transmission system, it is possible to collectively amplify WDM signal lights by an optical amplifier, to thereby permit realization of a lager capacity and a long distance transmission with a simple constitution (economical advantage).
In a conventional WDM optical amplifying-and-repeating transmission system, the cause of waveform distortion due to a nonlinear effect of optical transmission path includes self phase modulation (SPM), cross phase modulation (XPM), and four wave mixing (FWM). To reduce transmission characteristic degradation due to the nonlinear effect of optical transmission path, there is used a method for managing a wavelength dispersion of an optical transmission path.
For example, in an article of N. S. Bergano et al., xe2x80x9cWavelength division Multiplexing in Long-Haul Transmission Systems, IEEE Journal of Lightwave Technology, vol. 14, no. 6, pp. 1299-1308, 1996xe2x80x9d, there is used an optical transmission path combining a dispersion-shifted fiber (DSF) having a length of about 900 km and having a zero-dispersion wavelength of 1,585 nm with a single-mode fiber (SMF) having a length of about 100 km and having a zero-dispersion wavelength of 1,310 nm. This optical transmission path has an averaged zero-dispersion wavelength of about 1,558 nm, and accommodates wavelengths of signal lights ranging from 1,556 nm to 1,560 nm. The distortion of a transmitted waveform such as due to four wave mixing can be mitigated, because the wavelength dispersion of the DSF is about xe2x88x922 ps/nm/km, in which a group velocity of signal lights and spontaneous emission light and a group of mutual signal lights are different from each other so that an interaction period of time of nonlinear effect can be shortened.
Further, to realize a WDM optical transmission system having a more larger capacity and for a longer distance, the main subjects required for an optical transmission path of the system includes: (a) lower transmission loss; (b) a larger size of nonlinear effective cross section; (c) inconsistency of the zero-dispersion wavelength of signal light with that of the optical transmission path; (d) a relatively small absolute value of an accumulated wavelength dispersion; (e) substantially longer compensation intervals for the accumulated wavelength dispersion, as compared with repeating intervals; and (f) a smaller wavelength dispersion slope or an ability of compensating for the wavelength dispersion slope.
As a countermeasure for the aforementioned subjects, it has been proposed to use, as an optical transmission path, a combination of a 1.3 xcexcm zero dispersion fiber having a positive wavelength dispersion with a dispersion compensation fiber having a negative wavelength dispersion.
For example, in the system such as proposed in an article of M. Murakami et al, xe2x80x9cLong-haul 16xc3x9710 WDM transmission experiment using high order fiber dispersion management technique, ECOC ""98, p. 313, 1998xe2x80x9d, there are adopted a 1.3 xcexcm zero dispersion fiber having a positive wavelength dispersion and a dispersion compensation fiber having a negative wavelength dispersion, at the former half and the latter half of a transmission section, respectively. The optical fiber at the latter half is capable of compensating for the wavelength dispersion and dispersion slope of the optical fiber at the former half, the nonlinear effective cross section of the optical fiber at the former half is as large as about 80 xcexcm2, and the transmission loss of the transmission section is as small as about 0.20 dB/km. Thus, the accumulated wavelength dispersion included in the signal lights of all wavelengths can be sufficiently reduced, so that distortion of transmitted waveform due to self phase modulation-group velocity dispersion (SPM-GVD) can be reduced.
Meantime, when it is possible to reduce the distortion of transmitted waveform due to occurrence of self phase modulation and four wave mixing in case of adopting the aforementioned known type of optical transmission path, it is very possible that cross phase modulation becomes the dominant cause of transmitted waveform distortion due to a nonlinear effect of the optical transmission path. Namely, in an optical transmission path capable of compensating for the wavelength dispersion and its slope at particular intervals, the time-wise arrangement of WDM signal lights is restored at each dispersion compensating intervals, resulting in a possibility of larger waveform distortion due to cross phase modulation.
However, substantially no proposals have been done for conducting wavelength dispersion management taking notice of cross phase modulation, to thereby reduce the degradation of transmission characteristics due to a nonlinear effect of optical transmission path.
There will be briefly explained hereinafter the occurrence of transmitted waveform distortion due to cross phase modulation.
Generally, cross phase modulation is a phenomenon in which intensity fluctuation of another signal light is transformed into phase fluctuation of a pertinent signal light, due to a nonlinear effect of an optical transmission path. Intensity fluctuation of light transmitted through an optical fiber leads to a slight change of a refractive index of the optical fiber, due to a Kerr effect attributing to the optical fiber. The velocity of propagated light changes in accordance with the refractive index change, resulting in occurrence of phase fluctuation of the propagated light itself.
FIG. 15 is a view for explaining phase fluctuation due to self phase modulation and cross phase modulation, respectively. To simplify the explanation, it is assumed that an isolated light pulse is transmitted.
Firstly, for the phase fluctuation due to self phase modulation as shown in FIG. 15A, the light intensity of the isolated light pulse rapidly increases at the leading edge portion of the pulse, leading to occurrence of red shift due to the Kerr effect. This red shift is a phenomenon in which the wavelength of the signal light shifts toward a longer wavelength side, i.e., into a direction where the light frequency is reduced. The red shift is also called xe2x80x9cred chirpingxe2x80x9d. Contrary, the light intensity of the isolated light pulse rapidly decreases at the falling edge portion, leading to occurrence of blue shift due to the Kerr effect. This blue shift is a phenomenon in which the wavelength of the signal light shifts toward a shorter wavelength side, i.e., into a direction where the light frequency is increased. The blue shift is also called xe2x80x9cblue chirpingxe2x80x9d.
Next, for the phase fluctuation due to cross phase modulation, as shown in FIG. 15B, it is assumed that two signal lights (isolated light pulses) exist and the second signal light pulse is about to get ahead of the first signal light pulse. The reason why this situation is supposed is that, the propagation velocities of respective signal lights become different from each other due to the wavelength dispersion characteristics of an optical transmission path, if the wavelengths of the two signal lights are different from each other. As shown in FIG. 15C, in a state where the second signal light came close to the first signal light and has started colliding with the same, the light intensity is rapidly increased so that red chirping occurs. Further, as shown in FIG. 15D, in a state where the second signal light has gotten ahead of the first signal light and is about to leave the same, the light intensity is rapidly reduced so that blue chirping occurs.
If the power of the second signal light is constant while the second signal light has started getting ahead of the first signal light and has gotten ahead of the same, the red chirping and blue chirping due to the cross phase modulation will offset to each other. Actually, however, when, for example, the loss in an optical transmission path is to be compensated for making use of such as an optical amplifying-repeater, the signal light power frequently changes on the way of transmission. In such a case, red chirping and blue chirping may not offset to each other and either one of them may be left, depending on whether the collision of two light pulses occur in the higher light power state (such as just after the output of an optical amplifying-repeater or in the former half of a repeating section) or in the lower light power state (such as just before the input of an optical amplifying-repeater or in the latter half of a repeating section).
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide a wavelength dispersion compensating method for reducing the distortion of a transmitted waveform due to cross phase modulation which is one of the nonlinear effects attributing to an optical transmission path, as well as an optical transmission system using such a method.
To achieve the above object, according to one aspect of the present invention, a wavelength dispersion compensating method of a transmission path for transmitting wavelength division multiplexed signal light including two or more optical signals of different wavelengths, comprises the steps of dividing the transmission path into a plurality of repeating sections, and setting the amount or amounts of wavelength dispersion in one repeating section or multiple repeating sections in the transmission path, to be such a value or values that the amount of blue chirping and the amount of red chirping due to cross phase modulation to occur at least between light signals of adjacent two wavelengths substantially offset to each other.
According to such a compensating method, when the wavelength division multiplexed signal light is transmitted through one or multiple predetermined multiple repeating sections, time delays matching with one bit interval are caused between optical signals of respective wavelengths such that the red chirping and blue chirping due to cross phase modulation to occur upon collisions of light pulses are to substantially offset to each other. In this way, it becomes possible to reduce transmitted waveform distortion to be caused by cross phase modulation resulted from collision of light signals of adjacent two wavelengths.
Further, for the dispersion compensating method, the compensation of wavelength dispersion may be conducted such that blue chirping and red chirping occur once, respectively, in multiple repeating sections in the transmission path.
The transmission path attenuates light corresponding to a transmission distance of the light, optical amplifiers for amplifying the light attenuated in the transmission path are provided between the respective repeating sections, and the compensation of wavelength dispersion may be conducted such that the amount of blue chirping and the amount of red chirping due to cross phase modulation to occur between light signals of adjacent two wavelengths offset to each other, in multiple repeating sections in the transmission path. Further, when the amount of blue chirping and the amount of red chirping due to cross phase modulation to occur between light signals of adjacent two wavelengths are set to offset to each other in one repeating section in the transmission path, the compensation of wavelength dispersion may be conducted such that a plurality of cycles of occurrence of blue chirping and red chirping are provided.
Alternatively, the transmission path itself is capable of distributively amplifying the light being transmitted therethrough along a longitudinal direction thereof, so as to avoid the attenuation of light corresponding to a transmission distance thereof, and when the amount of blue chirping and the amount of red chirping due to cross phase modulation to occur between light signals of adjacent two wavelengths are set to offset to each other in one repeating section in the transmission path, the compensation of wavelength dispersion may be conducted such that blue chirping and red chirping occur once, respectively, in the one repeating section.
The transmission path in the wavelength dispersion compensating method preferably comprises an optical fiber having a positive value of wavelength dispersion and an optical fiber having a negative value of wavelength dispersion.
According to another aspect of the present invention, a wavelength dispersion compensating method for a transmission path for transmitting wavelength division multiplexed signal light including two or more optical signals of different wavelengths, comprises the steps of dividing the transmission path into a plurality of repeating sections, providing optical amplifiers between the respective repeating sections, for compensating for losses to occur in the transmission path, controlling the time delay amount between signal lights of adjacent two wavelengths to be caused by wavelength dispersion in one repeating section in the transmission path, to be smaller or greater than the bit interval at the transmission rate of the wavelength division multiplexed signal light; and conducting the compensation of wavelength dispersion such that the amount of blue chirping and the amount of red chirping due to cross phase modulation to occur between light signals of adjacent two wavelengths, in one repeating section or multiple repeating sections in the transmission path, offset to each other,.
The optical transmission system of the present invention is constituted by utilizing the aforementioned wavelength dispersion compensating method, and the concrete constitution thereof will be described hereinafter in detail with respect to the preferred embodiments.
Further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments when read in conjunction with the accompanying drawings.